The invention relates to axial-flow steam turbine wheels.
The blades of the wheels of axial-flow steam turbines are of twisted form typically where the ratio of blade height to stage diameter is large. Centrifugal stress elastically deforms the blades in the untwisting sense. Where the blades are relatively long and move at relatively high speeds, for example in the final low pressure stage of a large turbine, the centrifugal stress is relatively high and may cause blade tip sections to turn by as much as 10.degree. in the untwisting sense.
It is known to use shrouding or lacing on such blades to raise the fundamental frequency of vibration of the blades to inhibit flutter of the tip sections.
The tendency of the blades to untwist under the centrifugal stress causes stress and distortion of the shrouding or lacing which can lead the shrouding or lacing to fail. However, if the shrouding or lacing is strong enough to resist untwisting of the blades, torsional stress is also imposed on the blades in addition to centrifugal stress and vibration. Such a combination of stresses may well become unacceptable.